Why is BMI criticised so much if it is still used everywhere?

Because it was designed to describe populations, not individuals. It uses only height and weight, so a muscular person reads as overweight and someone with low muscle mass can read as healthy while carrying excessive body fat. It also shifts in meaning across ancestries, which is why some guidelines use lower thresholds for people of South and East Asian descent. It remains in use because it is cheap, reproducible and correlates well across large groups.

How accurate are calorie and TDEE estimates?

The equations behind them predict measured energy expenditure within roughly ten per cent for most people, which on a 2,200 calorie target is a couple of hundred calories either way. Activity multipliers add more uncertainty than the base equation does, because people consistently overestimate how active they are. Treat the number as a two-week starting point and adjust based on what actually happens to your weight.

Can I rely on a due date or fertile window from a calculator?

Only as an estimate. Due dates from a last menstrual period assume a regular 28-day cycle with ovulation on day fourteen, which is not true for many people, and only about one birth in twenty happens on the estimated date. An early ultrasound is substantially more accurate and is what clinicians use. Fertile window estimates carry the same caveat and are not a contraceptive method.

Health & Fitness calculators

BMI, TDEE, macro, keto, pregnancy, sleep, and wellness calculators

24 calculators · 5 guides

Health calculators are useful for tracking trends and setting a starting point, and unhelpful when treated as verdicts. A body mass index cannot tell muscle from fat, a calorie target is an estimate with a wide error bar, and a due date is a midpoint rather than an appointment.

Each page therefore explains the range your result falls in, what the number is actually measuring, and where the method breaks down. For athletes, older adults, children, during pregnancy, or with a medical condition. Use them to frame a conversation with a clinician, not to replace one.

Start with these in Health & Fitness

BMI CalculatorCalculate your Body Mass Index and see where you fall on the health scale.Health & Fitness TDEE CalculatorCalculate your Total Daily Energy Expenditure from three published equations and see what you burn at rest and through activity.Health & Fitness Macronutrient CalculatorCalculate your daily calorie target and protein, carbohydrate and fat grams from your body stats, activity level and goal.Health & Fitness VO2 Max CalculatorEstimate maximal oxygen uptake from a Cooper test, a Rockport walk, a 1.5 mile run or resting heart rate, and see where it sits for your age and sex.Health & Fitness Heart Rate Zones CalculatorWork out your five training heart rate zones using the Karvonen heart rate reserve method or a straight percentage of maximum.Health & Fitness Pregnancy Due Date CalculatorEstimate your due date and pregnancy milestones based on your last menstrual period.Health & Fitness

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24 calculators

B

C

D

Dog Age CalculatorConvert your dog's age to human years using modern formulas that account for breed size.

H

Heart Rate Zones CalculatorWork out your five training heart rate zones using the Karvonen heart rate reserve method or a straight percentage of maximum.

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K

Keto Macro CalculatorSet ketogenic macro targets from a fixed net carb limit, a protein target scaled to body weight, and fat filling the remaining calories.

M

Macronutrient CalculatorCalculate your daily calorie target and protein, carbohydrate and fat grams from your body stats, activity level and goal.

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P

R

Running Pace CalculatorCalculate pace, finish time, or distance for running.

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T

TDEE CalculatorCalculate your Total Daily Energy Expenditure from three published equations and see what you burn at rest and through activity.

V

VO2 Max CalculatorEstimate maximal oxygen uptake from a Cooper test, a Rockport walk, a 1.5 mile run or resting heart rate, and see where it sits for your age and sex.

W

Guides for Health & Fitness

Questions about health & fitness calculators

Why is BMI criticised so much if it is still used everywhere?: Because it was designed to describe populations, not individuals. It uses only height and weight, so a muscular person reads as overweight and someone with low muscle mass can read as healthy while carrying excessive body fat. It also shifts in meaning across ancestries, which is why some guidelines use lower thresholds for people of South and East Asian descent. It remains in use because it is cheap, reproducible and correlates well across large groups.
How accurate are calorie and TDEE estimates?: The equations behind them predict measured energy expenditure within roughly ten per cent for most people, which on a 2,200 calorie target is a couple of hundred calories either way. Activity multipliers add more uncertainty than the base equation does, because people consistently overestimate how active they are. Treat the number as a two-week starting point and adjust based on what actually happens to your weight.
Can I rely on a due date or fertile window from a calculator?: Only as an estimate. Due dates from a last menstrual period assume a regular 28-day cycle with ovulation on day fourteen, which is not true for many people, and only about one birth in twenty happens on the estimated date. An early ultrasound is substantially more accurate and is what clinicians use. Fertile window estimates carry the same caveat and are not a contraceptive method.